Viscosity index improvers are known to be added to lubricating oil compositions to improve the viscosity index of the lubricant. Typical viscosity index improvers include polymers of methacrylates, acrylates, olefins (such as copolymers of alpha-olefins and maleic anhydride and esterified derivatives thereof), or maleic-anhydride styrene copolymers, and esterified derivatives thereof. However, such viscosity index improvers can have poor shear stability, too high a viscosity at low temperature, poor fuel economy, and poor non-dispersant cleanliness.
U.S. Pat. Nos. 6,573,224; 6,174,843 6,419,714; and 4,526,950, and International Application WO 07/133999 all disclose olefin copolymers for lubricating compositions.
International publication WO2010/014655 A discloses a copolymer comprising units derived from monomers (i) an α-olefin and (ii) an ethylenically unsaturated carboxylic acid or derivatives thereof esterified with a primary alcohol branched at the β- or higher position, wherein the copolymer, prior to esterification, has a reduced specific viscosity of up to 0.08. The copolymer is useful to provide to a lubricant composition with at least one of acceptable or improved shear stability, acceptable or improved viscosity index control, acceptable or improved low temperature viscosity and acceptable or improved oxidation control.
Many lubricants contain sulphur-containing additives such as sulphurised olefins, alkyl sulphides, or various dimercaptothiadiazoles. These additives are known to provide extreme pressure performance in various mechanical devices such as driveline devices. However, the sulphur-containing additives may in some instances cause copper corrosion, or increased odour, or improved handling.